1. Field of the Invention
The present invention relates generally to wireless communications and, in particular, to a data transmission method and apparatus for maximizing the system throughput from multiple Collaborative Base Transceiver Stations (C-BTSs) to multiple Collaborative Access Terminals (C-ATs) based on a limited amount of feedback information in a collaborative Space Division Multiple Access (SDMA) Multiple Input Multiple Output (MIMO) system.
2. Description of the Related Art
MIMO has been provided to provide high speed and high quality data services in radio communication systems. MIMO technology operates with spatial processing based on the Channel State Information (CSI) between a transmitter and a receiver, such that the transmitter and receiver should know the CSI. Particularly in a downlink, the BTS should know the information on all of the channels between nT transmit antennas of the BTS and nR receive antennas of an Access Terminal (AT). In a Frequency Division Multiplexing (FDD) system in which the downlink and uplink are on different frequency bands, channel conditions between the downlink and downlink differ from each other such that the BTS depends on the downlink CSI that is estimated and fed back by the AT. Recently, codebook-based MIMO techniques have been provided to improve the spectral efficiency with a reduced amount of feedback information, rather than full CSI. The transmitter and receiver share a codebook of weight vectors, and the receiver estimates downlink channel and feeds back an index of the weight vector, maximizing the transmission performance to the transmitter.
Most conventional MIMO technologies have focused on how to avoid or suppress the intra-cell interferences, i.e., the interferences between data streams transmitted simultaneously in the same cell. Particularly in the conventional single cell SDMA techniques, each BTS forms as many beams as the number of transmit antennas in order to transmit data for the respective ATs. In this case, however, if the ATs receiving data through the Individual SDMA channels of respective BTSs are located in the overlapped service coverage area of the adjacent BTSs, inter-cell interference increases considerably, resulting in reduction of transmission data throughput of the system. Accordingly, there is a need to develop an improved SDMA technique for suppressing the Inter-Cell Interference (ICI) as well as intra-cell interference.
Collision Avoidance SDMA (CA-SDMA) is one of the more recently provided SDMA techniques in which each C-BTS selects a combination of precoding matrices from an SDMA codebook for a single cell and feeds back the selected precoding matrices combination along with Channel Quality Information (CQI). Each C-BTS selects the precoding matrices combination to be used by the C-BTSs that maximizes the C-BTS system transmission capacity and target C-ATs. That is, the CA-SDMA uses collaborative scheduling for suppressing ICI among the C-BTSs by preventing the collisions between the precoding matrices used by the C-BTSs while using codebook and transmission techniques for the conventional single cell SDMA. However, because the conventional SDMA technique developed for suppressing inter-cell interference is little different from the method for suppressing the intra-cell interference, this technique is not expected to suppress the inter-cell interference more effectively.
Accordingly, there is a need to develop a joint SDMA technique that is capable of maximizing simultaneous data transmission throughput from multiple C-BTSs to multiple C-ATs based on a collaboration of the C-BTSs and limited feedback information amount.